This invention relates to a semiconductor read only memory device and a method of manufacturing the same.
In the manufacture of a semiconductor read only memory (ROM), usually data is written in the ROM using a mask during the manufacture. Such a ROM is called mask program ROM. Data is written in the mask program ROM by using one of three, extensively employed data storage systems, i.e., a contact system, in which electrodes are selectively made in contact with memory cells, a SDG system, in which data is written depending on the presence or absence of transistor, and a system, in which the transistor threshold voltage is varied according to write data.
Meanwhile, ROMs are classified with respect to the circuit construction into those of a NOR type and those of a NAND-NOR type. The former ROMs are suited for high speed operation, and the latter for low speed operation.
With respect to the drive system, ROMs are classified into synchronizing ROMs and unsynchronizing ROMs.
The NOR-type ROM, suited for high speed operation, features readiness of circuit design and readiness and reliability of data writing. Further, the contact system is adopted for data writing, because a data writing step is in the latter half of the entire process of manufacture of the ROM.
In a ROM adopting the contact system, e.g., one disclosed in U.S. Pat. No. 603,698, where a wiring layer contiguous to the drain region of a MOS transistor is formed on a semiconductor substrate, a gate electrode is formed on the substrate before an insulation layer is formed on the substrate surface inclusive of the gate electrode. The insulation layer is formed with a contact hole for the drain region by photo-etching. A conductive layer is then formed on the insulation layer inclusive of the portion of the drain region exposed by the contact hole. The contact layer is patterned by photo-etching using a mask, whereby a contact electrode is formed in the drain region.
An insulation layer is formed on the entire surface of the semiconductor structure thus obtained. The insulation layer is selectively removed according to write data by photo-engraving process using a mask. A contact electrode contiguous to each intended drain region is thus exposed. Subsequently, aluminum wiring is connected to each exposed contact electrode to complete a ROM where data lines are formed.
In the above ROM, however, the processes of forming the contact electrode contiguous to each drain region and aluminum wiring require mask registration. The mask registration requires a certain margin. Therefore, when the number of mask registration processes is increased, it becomes impossible to realize a high density ROM.